1. Field of the Invention
The invention relates to periscopes for observing a field of view from within a protected structure and particularly to a periscope for armored vehicles. More particularly, the invention relates to a blackout blind mounted within the periscope for covering and exposing the eyepiece viewing opening to prevent light from being emitted from within the vehicle when the periscope is not in use.
2. Description of the Prior Art
Numerous periscope constructions have been developed and are in use for armored vehicles such as tanks. These periscopes contain various optical systems which enable the occupants to observe a field of view exterior of the vehicle without providing a direct opening through the armor for passage of projectiles, chemicals, liquids or other harmful materials. Many periscope constructions presently used on tanks, consist of a plastic block mounted within a protective housing which extends through an opening in the tank. The observer views an exterior field of view through a window opening in an end portion of the block. These periscopes permit light to pass from the interior to the exterior of a tank as well as in the reverse direction, when viewing an object in the field of view. This presents a serious problem in that such an internal light when emitted can be observed by an enemy and reveal the position of the tank. Modern light detection equipment in use by the military, is able to detect even small amounts of light in order to pinpoint an enemy's location. This problem requires eliminating or greatly reducing the amount of light possible within a tank, which light is necessary in many situations for map reading, etc., as well as for the general comfort and convenience of the crew.
Various shade and shutter constructions have been devised in an attempt to eliminate this problem. Makeshift flaps have been mounted above or adjacent the periscope viewing opening which are flipped up or down to cover the opening when not in use. These devices occupy critical space within the tank and are easily broken or damaged due to the severe shocks that the tank experiences. Also complete sealing and blackout is not always achieved.
Problems arose in attempting to mount a blackout blind within the interior of a periscope in that only a very small amount of space exists between the transparent periscope block and its protective housing. This limited space results in severe radii of curvature through which any such concealed blind must move when opening and closing the viewing window. The material required for the blind must be opaque and provide sufficient flexibility to move through such radii under operating conditions as low as -65.degree. F., and heat well in excess of 165.degree. F. without becoming brittle or weak. Likewise, the blind material should be self-lubricating such as plastic to eliminate the need of any external lubrication. However, the blind must not crack or take a permanent set when kept in a bent position of the severe radii in such extremely cold temperatures, as do many types of plastics. It also is desirable that such a blackout blind construction be able to be incorporated within existing periscope designs and constructions without major modifications.
No periscope construction of which I am aware eliminates these problems in a simple, effective and inexpensive manner by the use of a polycarbonate sheet movably mounted on a guide track within the periscope between the housing and transparent plastic block.